In principle, a condenser microphone comprises a thin membrane or diaphragm that is mounted in close proximity to a back plate. The thin membrane is fixed at its edges, so that it is able to deflect when sound pressure is acting on it. Together, the membrane and the back plate form an electric capacitor, where the capacitance changes according to said deflection of the membrane. In use, the capacitor will be charged using a DC voltage, usually called polarization or bias voltage. When the capacitance varies due to a varying sound pressure, an AC voltage that is proportional to the sound pressure will be superimposed on the DC voltage, which AC voltage is used as an output signal of the microphone.
The ever decreasing size of electronic devices has now lead to so-called MEMS (Micro Electro-Mechanical Systems) microphones, which mark a technical borderline what today is possible with respect to miniaturization of microphones. Those microphones have special requirements and need for special manufacturing processes. Micromachined microphones are usually developed for a lower voltage than standard microphones, normally for a voltage below 10V, which is why also the air gap is much smaller, usually less than 5 μm so as to obtain a sufficient field strength for an acceptable sensitivity of the microphone. However, the voltage cannot be increased to any value, because if the electric field strength exceeds a certain limit, the membrane snaps to the back plate which causes a short circuit. Of course this depends also on the stiffness of the membrane, which increases with thickness and Young's modulus of material used. On the other hand, smooth membranes at a high voltage are wanted for a high sensitivity or a low noise level respectively. Therefore designing a microphone means always finding the balance between a wanted sensitivity and the threatening collapse of the membrane. Hence the aforementioned concrete values for an air gap and a polarization voltage only serve as an example, just for illustration of the dimensions of a MEMS microphone.
FIG. 1a shows a cross section of a prior art MEMS microphone 1. First of all a silicon die 3 is coated with a conductive layer, which forms the membrane 2′. After this coating a cavity is etched into the die 3, thus freeing the membrane 2′. On top of this construction is placed a back plate 4 comprising holes 5, wherein an insulator 6 electrically separates the membrane 2′ from the back plate 3. Optionally, the membrane 2′ is made of an insulator. In this case a conductive layer on or under the membrane is used as an electrode. This conductive layer may also serve as an shielding against electromagnetic interference.
In use a polarization voltage is now applied to the membrane 2′ and the conducting back plate 3, thus mechanically preloading and therefore bending the membrane 2′. The bold-line membrane 2′ in FIG. 1b indicates the idle position IDL after biasing the system by means of a polarization voltage. Varying air pressure in front of or behind the membrane 2′ caused by sound waves leads to a further bending of the membrane 2′. Thin lines indicate the upper and lower dead center UDC and LDC of the membrane 2′ for a given sound pressure. It should be noted that there is no translatory movement involved. The three positions of the membrane 2′ are separated for better visualization. In reality the outer area of the membrane is fixed and does not move so that there is only a bending within the membrane 2′.
The holes 5 in the back plate 3 serve as a necessary ventilation. Otherwise the up-moving membrane 2′ would compress the air between membrane 2′ and back plate 3, which would hinder the movement of the membrane 2′.
This prior art construction has some drawbacks: First, a certain stress within the membrane 2′ is a result of the production process. Unfortunately, this stress is neither predictable nor adjustable so that the idle position of the membrane 2′ is unknown as well. As a consequence, there is a great spread of the sensitivity across a couple of different microphones. Therefore the membrane 2′ may have a couple of small holes in the outer area, thereby decreasing the stress within the membrane 2′. FIG. 2a shows a top view of such a membrane 2′, wherein in the upper left corner the back plate 4 with holes 5 and in the lower right corner the membrane 2′ with holes 7 is shown. FIG. 2b shows an corresponding cross sectional view B, B′ of the microphone 1 which is quite similar to the one shown in FIGS. 1a and 1b. Of course the size of the holes 7 may not exceed a certain diameter because otherwise the ventilation through these holes 7 is too high, thereby decreasing the sensitivity of the microphone 1. In some solutions therefore these holes 7 are sealed again with a different material, which does not influence the stress within the membrane 2′ but only closes the holes 7.
Second, due to the construction the membrane 2′ only bends when different air pressures acts on it. Hence the center area contributes much more to an AC signal than the outer areas, which are more or less wasted.